This paper reports crystal structures, magnetic properties and thermal stability of TbCu7-type Sm8.5Fe(85.8-x)Co4.5Zr1.2Nbx (x = 0-1.8) melt-spun compounds and their nitrides, investigated by means of X-ray diffraction, vibrating sample magnetometer, flux meter and transmission electron microscope. It is found that the lattice parameter ratio c/a of TbCu7-type crystal structure increases with Nb substitution, which indicates that the Nb can increase the stability of the metastable phase in the Sm-Fe ribbons. Nb substitution impedes the formation of magnetic soft phase alpha-Fe in which reversed domains initially form during the magnetization reversal process. Meanwhile, Nb substitution refines grains and leads to homogeneous microstructure with augmented grain boundaries. Thus the exchange coupling pining field is enhanced and irreversible domain wall propagation gets suppressed. As a result, the magnetic properties are improved and the irreversible flux loss of magnets is notably decreased. A maximum value 771.7 kA/m of the intrinsic coercivity H-cj is achieved in the 1.2 at% substituted samples. The irreversible flux loss for 2 h exposure at 120 degrees C declines from 8.26% for Nb-free magnets to 6.32% for magnets with 1.2 at% Nb substitution. (C) 2018 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.